Well, I have the answer. It will not work, at least when in-place upgrade is. And, I suspect no way it will work, regardless of the way of upgrading to 4.2, when Xen PV disk drivers are installed in Win S-HVM. If I knew this, I’d backup the one working under 4.1, then uninstall Xen PV disk drivers, then back that version up, too, then upgrade to 4.2, then restored both (qwt<=4.1.69-1). The second one should work, while being able to attach external storage via qvm-usb. Which makes it unusable for non-primary internal storages.